1. Field of the Invention
This invention relates generally to an optical demodulation system employed in an analog optical link and, more particularly, to a feed-forward optical frequency demodulation or phase demodulation system including both coarse and fine demodulators and employed at an end of an analog optical link.
2. Discussion of the Related Art
Analog optical links are used in various optical communications systems where the transmission of large bandwidth signals are required, without the need for analog-to-digital (A/D) converters or digital-to-analog (D/A) converters. These analog optical links transmit RF signals modulated onto an optical carrier signal. The optical carrier signal generally is transmitted along a fiber optic cable or through free space to a receiver where it is demodulated to recover the RF data. The optical link allows the RF data to be transmitted with low losses and at high bandwidths, and thus is attractive in many communications systems to provide the desired performance, especially high frequency RF communications systems that transmit signals in the GHz bandwidth range. Also, telescopes used to transmit optical signals in free space have a much greater directivity than RF antennas of comparable size.
To have the desired performance for various communications systems, the optical link must provide a good dynamic range, i.e., allow the simultaneous transmission of signals having widely varying amplitudes that do not interfere with each other, and with minimal optical power requirements. Currently, intensity modulation (IM) is the dominant optical modulation choice for analog optical links. In IM, the intensity of the optical light is modulated with the RF signal. Unfortunately, IM does not provide high enough performance because significant transmission power is required to provide the desirable dynamic range and signal-to-noise ratio (SNR) for a particular application. In fact, ideal linear IM requires 9 dB more received optical power than ideal suppressed carrier amplitude modulation (AM) to get the same demodulated SNR. To overcome this problem, known intensity modulation optical links provide a series of optical amplifiers to boost the optical carrier signal power as it propagates along an optical fiber. The number of optical amplifiers needed can be costly. Also this technique cannot be used for free space links.
Wideband frequency modulated (FM) or phase modulated (PM) optical links can theoretically use the extremely wide bandwidth available at optical frequencies to achieve much better dynamic range and SNR than IM optical links for the same received power. For example, phase modulation having a peak phase deviation of 10 radians has a 26 dB greater link SNR potential compared to ideal IM, and a 17 dB greater SNR potential than suppressed-carrier AM.
Known FM or PM communications systems must significantly modulate the carrier frequency or phase to achieve better dynamic range and SNR performance than AM. In other words, the frequency deviation or phase deviation of the carrier signal which is induced by RF input signal must be large enough to increase the bandwidth of the modulated carrier substantially beyond that of an AM modulated carrier.
For FM or PM links using RF carrier signals, two basic demodulators are used. Both provide high dynamic range and SNR performance. The most basic demodulator uses an RF frequency discriminator followed by an RF envelope detector. The more complex demodulator uses a phase lock loop, but has the advantage of a lower threshold SNR. The threshold SNR is the received SNR in the transmission bandwidth above which the demodulator performs well.
The basic RF demodulator concept cannot be used when the carrier signal is optical, because a true optical envelope detector does not exist. The closest equivalent optical demodulator is an optical filter with a linear frequency-to-intensity transfer function followed by a photodetector which converts the intensity into an electrical current. One of the best implementation uses a dual-output unbalanced Mach-Zehnder interferometer (MZI) followed by a balanced photoreceiver. This demodulator works well when the frequency or phase deviation is small. However, when the frequency or phase deviation is large, it generates excess noise, causing the noise floor to rise significantly. A fast optical limiter is needed to prevent this noise floor rise. This demodulator also has significant third-order spurs and distortion which limit its dynamic range. The practical performance of this type of analog optical link is therefore not substantially better than known IM links.
Hybrid RF/optical demodulators with feedback, patterned after the RF phase lock loop, may provide high dynamic range and SNR performance for moderate RF signal bandwidths. But the time delays in the feedback path limit their performance for RF signals with bandwidths greater than a few GHz.
What is needed is a demodulation system to be used in connection with an analog optical link that provides suitable wideband FM or PM dynamic range and SNR potential, and handles large RF signal bandwidths. It is therefore an object of the present invention to provide such a demodulation system.